The present invention relates to function generators and, in particular, to a programmable function generator wherein signals defining the line segments simulating the function are generated in a simplified manner and the parameters for each segment are completely programmable.
A function or waveform generator is an electrical device which constructs or simulates an electrical waveform in response to one or more input signals. Such devices have found a variety of different applications in the electronics field. For instance, a function generator can be used to generate analog control waveforms and, thus, can be used as a central control mechanism for any voltage controlled device or system, such as a video or audio synthesizer. When used in conjunction with such a system, the device enables the operator to automatically implement a complex, predetermined series of events. For instance, the output of the function generator may be used to control an electron beam in a cathode ray tube or the like.
Many function generators can be partially or completely programmed to produce asymmetrical or other variant waveform outputs. Prior art examples of such programmable waveform generators include diode-type function generators, servo motor systems using cams and drums and digital memory systems.
Diode-type function generators are difficult to program and do not have independent adjustments at each programming point. Furthermore, while they are capable of responding to input signals relatively fast, they are sensitive to temperature variations. Servo motor systems are usually large and expensive, consuming a considerable amount of power while responding slowly to the input waveform. Digital systems also have numerous drawbacks. They are usually expensive because the input information must first be converted from analog to digital form, processed, and then reconverted to analog form. Thus, they require analog-to-digital and digital-to-analog converters, as well as a digital memory unit. Furthermore, this mode of operation often results in an output which varies in discrete steps. Thus, these types of devices tend to be overly complex and, because of discontinuities in the generated waveform, may not be suitable for certain applications.
One digital system is disclosed in U.S. Pat. No. 4,064,423 to Atkisson, Jr. In that patent, data for determining the characteristics of the line segments is stored in digital form in a programmable digital memory. The characteristics of each segment are read from the memory, in order, under the control of an address counter. The stored amplitude and time data is processed in a rate multiplier and a dividing counter to provide digital interpolations between the end points of the line segments. The multiplier has two portions. The first portion generates an output (base rate) which is utilized as a reference in resetting the system at the end of each line segment. The second section produces an output signal consisting of a number of pulses equal to the product of the base rate and the amplitude data.
The output of the second section is connected to the clock input of a counter which divides the signal from the rate multiplier by the time change information stored in the memory for each line segment. The output of the counter is divided into a series of digital pulses which are used to clock an up/down counter and determine the number of steps in each line segment. The stored information in the memory controls the counter and determines the amplitude of the segment. The output of the counter is a digital signal which drives a conventional digital-to-analog converter. The converter is supplied with a single reference signal and the output of the converter is a digitally-controlled percentage of the reference signal.
Since Atkisson utilizes a conventional single reference digital-to-analog converter, the digital control input thereto must be a complex signal containing the information relating to the duration, as well as the initial and termination points for each line segment. Because the control signal input to the converter must contain a large amount of information, the circuitry required to develop these complex digital control signals must be quite sophisticated.
The complexity of the Atkisson system and similar systems is a result of the type of voltage transition circuit which is used to generate the analog signal representative of the desired function. These devices utilize standard digital-to-analog converters which generate an analog signal which is a portion of a single fixed reference signal, the portion being determined by the digital control inputs. Thus, the digital control inputs must contain virtually all of the information concerning the line segment which the generated analog output represents.
It has been found that the necessary function can be generated in a far simpler and more eloquent manner and still permit the parameters of each of the line segments in the function to be independently programmed. This is possible due to the use of a unique voltage transition circuit which has first and second reference voltage inputs, an analog output and a digital control input. The first and second reference voltage inputs respectively receive voltages representative of the initial point and termination point of the line segment. The digital control signals represent the duration of the line segment and control the proportions of each of the separate input reference voltages which go to make up the analog output signal, at any particular time. Because the digital control signal inputs to the voltage transition circuit need not contain information concerning the end points of the line segment but only the duration thereof, the generation of the control signals is a relatively simple, straight-forward process, not requiring programmable electronic digital memories, analog-to-digital converters, rate multipliers, or complex counter systems.
It is, therefore, a prime object of the present invention to provide a programmable function generator wherein the parameters of the line segments are completely programmable.
It is another object of the present invention to provide a programmable function generator which is capable of cycling through a function at any desired repetition rate.
It is a further object of the present invention to provide a programmable function generator which utilizes a unique voltage transition circuit.
It is a further object of the present invention to provide a programmable function generator wherein the digital control signals for the voltage transition circuit are generated in a straight-forward, simple manner.
It is another object of the present invention to provide a programmable function generator wherein the object analog signal has improved resolution.
It is still another object of the present invention to provide a programmable function greater wherein the duration of each line segment may range from a fraction of a second to hours in length.
It is still a further object of the present invention to provide a programmable function generator which utilizes relatively simple components which interact reliably with a minimum of maintenance.
In accordance with the present invention, apparatus is provided for generating a signal representative of a function or the like consisting of a sequence of line segments. The apparatus comprises first programmable input means for defining a first parameter corresponding to each of the line segments. Second programmable input means generate a reference signal representative of a second parameter corresponding to each of the line segments. Means are provided for storing the generated reference voltages. Means are provided for generating control signals representative of each of the first parameters. Means are provided for generating an analog output signal comprising portions representative of each of the line segments. Each of the output signal portions is generated over a period determined by the control signal corresponding to the line segment which the portion represents, has an initial value determined by the stored reference voltage corresponding to the previous line segment, and a final value determined by the reference voltage for the line segment which the portion represents.
The first programmable input means preferably comprises a power source, a first variable resistance means corresponding to each of the line segments, and analog switch means having data input means for each of the line segments, a single data output means, and address signal input means. Each of the first resistance means is operably connected between the source and a different one of the input means of the analog switch means. The analog switch means serves to connect the appropriate resistance means to the data output means in accordance with an address signal corresponding to the line segment being generated. The output means of the analog switch means is connected by means of a time bus to the control signal generating means.
The first programmable input means also comprises means for generating address signals, connected to the analog switch means address input means and effective to cause the analog switch means to operably connect each of its input means to its output means, in a given sequence, such that each line segment is generated in turn. The address signal generating means preferably comprises a shift register and encoding means. The shift register drives the encoding means in accordance with a clock signal which, in turn, generates the necessary address signals for the analog switch means.
The second programmable input means preferably comprises a power source, a second variable resistance means corresponding to each of the line segments and a set of second analog switch means having data input means for each line segment, a data output means for each line segment, and address signal input means. Each of the second resistance means is operably connected between the source and a different one of the input means. Each of the output means is connected by means of a voltage bus to the output signal generating means. The second programmable input means is also controlled by the means for generating address signals. This means is connected to the address input means of each switch in the set of second analog switch means and is effective to cause the second analog switch means to operably select particular input means for connection with the output means.
The reference voltage storing means preferably comprises a sample and hold circuit. When actuated by a strobe signal, the sample and hold circuit stores the reference voltage relating to the termination point of the previous line segment. This stored value is utilized as the initial point for the next line segment.
The control signal generating means comprises clock means and counter means. The clock means indexes the counter means at a rate in accordance with a timing signal and the output of the first programmable input means from the time bus. The output of the counter means comprises the signals which control the voltage transition circuit. The clock means preferably comprises a controllable frequency divider.
More specifically, the clock means preferably comprises a voltage controlled oscillator. The oscillator comprises a counter, means for indexing the counter at a predetermined rate in accordance with the timing signal, means for generating a ramp signal in accordance with the output of the counter, and means for comparing the generated ramp signal with the output of the first programmable input means appearing on the time bus. The comparing means comprises a means for generating the pulse when the ramp signal exceeds the output of the first progammable input means appearing on the time bus. The counter means is reset in accordance with the pulse.
The output signal generating means comprises a voltage transition circuit having first and second analog inputs respectively connected to receive the outputs from the second programmable input means appearing on the appropriate voltage bus, and from the storage means. The voltage transition circuit generates an analog output signal representative of a proportional mixture of the reference voltage inputs, determined in accordance with the control signals.
The voltage transition circuit comprises analog switch means comprising first and second sets of inputs, operably connected to said first and second analog inputs, respectively, a set of outputs and control inputs. The control inputs are operably connected to receive the control signals such that the analog switch means causes selected inputs from the input sets to be connected to its output in accordance with the control signals. The analog signal generating means also comprises means for summing the analog switch means outputs to produce an analog output signal. The summing means preferably comprises a resistance ladder.
In the preferred embodiment disclosed herein, the first parameter relates to the duration of the line segment. The second parameter relates to an end point of the line segment, and, more particularly, to the termination point thereof. Since the duration and termination points of the line segment are completely programmable, and the initial point is determined by the termination point of the previous line segment, which is also programmable, the characteristics of each line segment may be completely programmed, giving the apparatus of the present invention a great deal of versatility, notwithstanding the fact that the apparatus is relatively simple in function and structure.